The soil environment tells you when and where to apply this essential nutrient

Life isn’t fair—not with phosphate fertilizer, anyway. Timing and placement of phosphorus (P) applications—a critical part of crop production—is easier for a central Illinois farmer in a corn/soybean rotation than for his neighbor who grows continuous corn, or for a North Dakota farmer who has a shorter growing season and colder soils.

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Timing and placing P applications might even be simpler in one part of a field than another, if a field contains different soil types. That’s because P management must be based on the soil environment—moisture, temperature, crop residue, calcium level and pH.

"Wherever you farm, whatever your soil types, your goal is to have phosphorus available throughout the vegetative life of the plant, especially the first six weeks when yield potential is determined," says Farm Journal Field Agronomist Ken Ferrie. "Phosphorus is critical for cell development and growth."

The availability of soil P is controlled by soil microorganisms, and the soil environment affects microbial populations. "By decomposing crop residue, microbes convert phosphorus from the unavailable organic form to the inorganic form that plants can use," Ferrie says. "It’s part of what we call the phosphorus cycle. The health­ier the microbial population, the easier it is to release phosphorus."

Extreme moisture conditions can reduce decomposition. Microbial activity declines during periods of drought. In saturated soil, there’s no oxygen for the microorganisms to breathe.

Soil temperature. When soil temperature is below 60°F, P availability is limited because microbes are not very active. "Because microbes use phosphorus for energy, as well as releasing it from crop residue, a positive balance of available phosphorus doesn’t begin to show up until the soil temperature reaches about 65°F," Ferrie says. "Until that time, there isn’t much phosphorus available to plants, even if the soil test reveals optimum levels, because the phosphorus remains unavailable in the fixed or organic form."

Because corn seed can germinate at 50°F, that can create a problem. "The young plants may spend lots of their early growth in soil with temperatures from 45°F to 60°F," Ferrie says.

"Depending on where you farm, soil moisture and temperature conditions have a big effect on early growth. In Minnesota, farmers plant as early as possible so their crop will mature before frost. They can’t wait for 60°F soil temperatures; they have to plant at 50°F. Not much P will be released from the soil until the soil temperature reaches 65°F. In Louisiana, that won’t be a problem because farmers plant into 60°F to 65°F soil temperatures.

"As a result, farmers in colder climates are more likely to see a yield response to phosphorus applied in starter fertilizer," Ferrie continues. "When we band phosphorus fertilizer next to the plant, we apply a water-soluble form, which does not depend on temperature or microbes to
become available. It supplies the crop with P until the microbes become active and start releasing phosphorus."

Paying the carbon penalty. Crop residue, which is determined by crop rotation, affects P availability by creating what Ferrie calls the carbon penalty. Think of residue as a load of carbon being applied to soil, which microbes must decompose. (The same kind of carbon penalty exists for nitrogen and sulfur, which also must be converted from organic to inorganic forms by soil microbes.)

"Both plants and microbes need phosphorus as a source of energy," Ferrie says. "As soil warms up, microbes become active and start to release phosphorus by decomposing crop residue. This is called mineralization. But they also consume phosphorus for energy to drive the process, making it temporarily unavailable to plants. This is called immobilization. Whether we have net mineralization or net immobilization depends on the amount of residue to be decomposed and its carbon to phosphorus (C/P) ratio."

If residue has a low C/P ratio, cycling P from residue into the soil nutrient pool is fairly easy and fast. If it has a high C/P ratio, cycling of P is much more complicated. "The more complicated the process, the more phosphorus is immobilized," Ferrie says. "So your crop rotation plays into timing of phosphorus fertilizer.

"With continuous corn, which has a higher C/P ratio, more immobilization occurs in the spring. So continuous corn growers face more availability issues than growers in a corn/soybean rotation. That’s why we see a good response to starter fertilizer in continuous corn.

"If the carbon/phosphorus ratio is less than 200:1, there will be a net gain of available phosphate in the soil because of mineralization," Ferrie summarizes. "If the ratio is between 200:1 and 300:1, you break even. If the ratio is greater than 300:1, there will be a net loss of available phosphorus through immobilization."

The amount of immobilization and mineralization will vary, depending on the C/P ratio of the previous crop and your climate. "In the southern U.S., with residue decomposing during the winter, you have fewer phosphorus issues in the spring," Ferrie says. "In the north, you must decompose all of the residue in the spring. The slower your soil warms up, the less available phosphorus you will have."

Tillage affects the carbon penalty. "If you chisel in the fall and have warm temperatures to help decompose the residue, you will have fewer phosphorus availability issues than if you chisel in the spring and incorporate residue just ahead of planting," Ferrie says. "With spring tillage, as the soil warms, microbes will draw down your supply of available soil phosphorus.

"If you leave residue on top and strip-till or no-till, you’ll have a smaller carbon penalty," Ferrie continues. "That’s because stalks, cobs, husks and shanks on the surface decompose more slowly than when they are incorporated. In continuous corn, if you see stalks and cobs from the previous year lying on the surface, they are not part of the carbon penalty because they are not decomposing."

Tillage also impacts soil temperature, which determines how soon soil microbes become active in the spring. "The faster soil warms up, the sooner the microbes start cycling phosphorus," Ferrie summarizes. "Because no-tilled and strip-tilled soil takes longer to warm up, strip-till farmers often see a yield response when they apply phosphate as they build their strips in the fall."

Manure also influences immobilization and mineralization. "If your manure contains cobs and stalks that were used for bedding, their high carbon/phosphorus ratio can cause early season phosphorus immobilization," Ferrie says. "Unless the manure contains enough phosphorus to break down those cobs and stalks, you will need to apply phosphate fertilizer to help the microbes discompose them and maintain a positive balance of available phosphorus."

Fixation and pH. Other aspects of the soil environment are calcium and pH. "In soils with high calcium content, phosphorus anions, which have a negative charge, bond to calcium cations, which have a positive charge," Ferrie says. "When phosphorus becomes ‘fixed’ to calcium, it creates dicalcium phosphate or tricalcium phosphate. The phosphorus is held so tightly that it is effectively lost forever to crop production."

There’s no way to change a high-calcium soil, so you have to adjust your fertilizer management. "In high-calcium soils, try to apply phosphate fertilizer as close to the time of usage by the plant as possible, so the root system picks up the phosphorus before it bonds with calcium," Ferrie advises. "Do not apply phosphate fertilizer in the fall; instead, apply it ahead of planting in the spring. Try to avoid broadcasting in favor of banding. The more years you band phosphorus in a field with availability issues, the more availability you will gain."

Acid soils also have P fixation issues because phosphorus bonds with iron and aluminum in the soil solution. "Iron and aluminum phosphates don’t even show up on soil tests," Ferrie says. "Phosphorus will drop, not from crop removal but because it became unavailable. To deal with fixation, you can use the same practices as for calcaric soils; but applying lime is a better investment because it corrects the acidity, which stops fixation from occurring."

With P timing and placement, one size does not fit all. "The more the soil environment works against you for early or season-long uptake, the more aggressive you must be with timing and placement," Ferrie concludes.